The present invention lies in the field of additives to wellbore fluids used while drilling wells in earth formations, completion operations after the drilling has been completed, workover, fracturing, and various other operations is a wellbore. More particularly, the invention is concerned with additives to non-clay wellbore fluids such as pure water, various brines and emulsions of water and oil. The additives of the invention include two parts, a basic additive used to control one or both the viscosity of the fluid and its fluid loss properties, and a catalyst or stabilizer which enhances the effect of the basic additives.
Quite frequently clay-based drilling muds, such as, those described in U.S. Pat. Nos. 1,856,356 to Weiss et al and 2,868,726 to Bunker et al. have employed a common water loss additive such as pre-hydrolyzed starch or carboxy-methylcellulose (CMC). Carboxy-methyl cellulose as such is a non-ionic water insoluble material. As a result CMC or carboxymethyl cellulose is employed as a soluble salt of carboxymethyl cellulose. The use of these salts as water loss additives has presented quite a problem as denoted in U.S. Pat. No. 3,625,889 to Branscum. The presence of even small amounts of water soluble inorganic compounds, such as calcium chloride will expel CMC from the solution. For that reason CMC has never been satisfactory for use with non-clay fluids because in the non-clay fluids water soluble inorganic salts are employed to develop the desired density.
Non-clay wellbore fluids are desirable where, for example, the build-up of a filter cake is undesirable. Moreover, the heaving shale materials or clays in contact with the clay-based drilling fluid results in incidental and undesirable fluctuation of the properties of the drilling fluids. Removal of these extraneous materials from the clay-based mud is especially difficult because of the similarity of the extraneous material and the clay base. In the non-clay based wellbore fluids, such solids are easily removed from the fluid without removal of any of the drilling components because of the dissimilarity of the materials, i.e., extraneous materials and wellbore fluid.
Various methods have been proposed for overcoming the incidental mud-making aspects of clay-based drilling fluids. The Weiss et al. patent dislcosed that very high magnesium ion content, that is, at least 400 ppm of magnesium ion in the drilling fluid will prevent or at least reduce incidental mudmaking by stabilizing or hardening the heavy shale material or clays in contact with the drilling fluid. One manner of achieving this high magnesium ion content proposed by Weiss et al. is the use of an exotic MgO described as "hydratable magnesium oxide", which is obtained only by low temperature preparation as opposed to commonly available high temperature MgO known as magnesia which is essentially non-hydratable. The solubility of the hydratable MgO is further enhanced by soluble ammonium salts in order to totally solubilize the MgO and effect the high magnesium ion content.
Heaving shale and clays in the wellbore do not present the same type of problem to non-clay based fluids since the solids are easily removed without changing the nature of the wellbore fluid, and a magnesium ion content is not necessary or particularly desirable.
When a non-clay wellbore fluid is used for a purpose requiring relatively high viscosity, for example drilling or milling, it is generally necessary to employ an additive to thicken the fluid to the point where it will have the necessary carrying capacity. Several additives to increase viscosity are commercially available, but most if not all of these have one or more limitations. The viscosifier may be slow to yield, i.e., it may take 15 minutes or more from the time of addition to the time when the fluid becomes thick enough to carry the cuttings. The additives may be effective over only a narrow low temperature range, permitting the fluid to thin out again when a higher temperature is reached. In addition, most of the viscosifiers have a limited service life, again thinning out after a period of use.
A class of compounds that has now been recognized as being suited for providing non-structured viscosity to non-clay wellbore fluids are the hydroxyalkyl cellulose. These materials such as hydroxyethyl cellulose (HEC) and hydroxypropyl cellulose are water soluble and non-ionic, thus they are not susceptible to being expelled from a brine solution, for example as are the soluble salts of carboxy-methyl cellulose. The term "non-structured viscosity" as used here means one wherein viscosity is obtained by physio-chemical rather than by physical means. Asbestos and attapulgite are examples of the types of materials employed to obtain structured viscosity.
The non-structured viscosity provides another unique benefit in that the carrying capacity will vary in the agitated and non-agitated states, so that when, for example, the agitation is reduced in a separating tank the carrying capacity will drop off and the cuttings and the like from the wellbore will fall out of the fluid, yet when agitated there is ample carrying capacity to carry the cuttings and the like to the surface from the wellbore.
Unfortunately, hydroxyalkyl cellulose is unstable in boreholes and has been relegated to the task of a water loss additive in clay-based fluids, such as those shown in U.S. Pat. No. 2,570,947 to Himel et al.
It is a general object of the present invention to provide additives for wellbore fluids in which the above enumerated disadvantages of commercially available viscosifiers will be eliminated or reduced. Stated positively, this general object is to provide additives for non-clay drilling fluids which are stabilized to provide faster yields, effectiveness over a higher temperature range, and effectiveness over a longer time period.
Commercially available additives for non-clay drilling fluids for the purpose of controlling loss of filtrate to the formation, whether alone or for the additional property of controlling viscosity, also have a disadvantage, namely that the fluid loss they permit is not as minimal as might be desired. It is therefore a second general purpose of the present invention to stabilize such additives to increase their effectiveness in reducing the amount of fluid lost to the formations to which the fluid is exposed.
The additives of the present invention are also designed for use in closed circuit drilling fluid circulation systems avoiding the many disadvantages of clay-based drilling mud, e.g., hydration and disintegration of drilled solids and the many disadvantages flowing therefrom such as overly large increases in unit weight and viscosity, build-up of an overly thick filter cake on the bottom and sidewall of the hole, swabbing and caving in of the cake and sidewall, and the necessity for the mud engineer to discard a part of the overladen fluid, thin out the rest and replace some of the additives discarded with the diverted fraction. The above and other desirable objects and advantages are achieved according to the present invention by stabilizing various commercially available additives with a material acting in the nature of a catalyst.